Teenagers will always test boundaries, and will always - at some point - try to be different (from you but not, of course, from one another - as a group, they're incredibly homogenous).
And some, though not all, will get into trouble along the way.
It may be just a bit of trouble, or it might be a major problem involving the police or a pregnancy or a fight. But when the crisis happens your job will be to be there for them, to help them realise that they will get through the worst of times; and to provide a lot of practical support as well.
Stealing and shoplifting
If the trouble involves illegal behaviour, you have to make a firm stand. Sometimes, kids start stealing from their parents - and some shoplift or steal from others outside the family. These Mumsnetters say:
"It is very common to steal from parents. It's bad but it is common. If money is around it can prove too tempting to a lot of teenagers. Teenagers often only think of themselves, it's 'normal' for them. If you are sure, though, you need to give some consequences such as dock it from her allowance/grounded or whatever."
"Do the 'I need to have a talk with you' routine, tell her what you've found and ask her to tell you what's going on. Don't enter into discussions if she comes up with daft excuses and don't get angry, just be serious. Explain that it's stealing, it's a crime, she could get a criminal record which might prevent her getting a good job (or even a holiday job). Whatever it was that made her do it (wanting the earrings, wanting her ears pierced, being bored, wanting to show off to her friends, whatever), stealing is never a good solution to that."
How to keep your teenager out of trouble
Research suggests that one of the best things parents can do to reduce the risks of their child getting into trouble is, quite simply, not to expect it. Where there are expectations of negative behaviour, these expectations are frequently played out. So have high standards, expect good behaviour, and you will be more likely to get it.
As far as you can, encourage extracurricular activities - sports, music, Duke of Edinburgh courses. These give youngsters an outlet for their energy, and are an important arena in which to work hard and succeed. Busy teenagers are less likely to be troublesome teenagers.
Encourage friendships with youngsters you regard as 'positive' peers. Your instincts are likely to be correct on who to encourage and discourage - although bear in mind that any whiff of disapproval from you, and the friend in common will rocket to the top of the list of favourites. So say nothing about kids you don't like the look of, and do everything you can to shore up the friendships with those you feel happy about (invite them round, get to know their parents etc).
Making your house a welcoming place for your teenager to bring friends is important. Welcome them in, but make it on your terms. If you're worried about drinking or smoking, don't allow it, but do provide soft drinks, suitable videos, snacks and space where they can chill and not be disturbed by you or (especially) younger siblings.